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How can Blu-Ray discs store so much more data than a DVD that is exactly the same size?

Blu-Ray disks can store about five time the amount of data that a DVD can store (5GB vs 25GB for a single layer disc), while visually appearing practically identical. I thought it might be layers, but that doesn’t seem to be it, since you have dual and single layer discs in either format. What’s actually different about Blu-Rays that allow them to store so much more data in the same amount of physical space? Are there any reliability advantages or disadvantages?

"While current optical disc technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM rely on a red laser to read and write data, the new format uses a blue-violet laser instead, hence the name Blu-ray. Despite the different type of lasers used, Blu-ray products can easily be made backwards compatible with CDs and DVDs through the use of a BD/DVD/CD compatible optical pickup unit. The benefit of using a blue-violet laser (405nm) is that it has a shorter wavelength than a red laser (650nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it's possible to fit more data on the disc even though it's the same size as a CD/DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Discs to hold 25GB/50GB. Recent development by Pioneer has pushed the storage capacity to 500GB on a single disc by using 20 layers. "

The disc is physically the same size, but the rest of the standard is quite different. The wavelength of the laser is completely different (hence the name “blue” ray), as is the read angle. The difference in wavelength is the key to the increased storage capacity. There are grooves on the bottom layer of the disc that the laser passes through to read data. The size of the grooves is dictated by the wavelength of the laser, and since the blue laser of Blu-Ray has shorter wavelength than the red laser of DVD, the grooves can be much closer together, so that data can be packed much more densely.

There also may be hardware service life advantages for Blu-Ray as well, since it doesn’t have to spin as fast as DVDs, but I’m not sure if anyone has ever done a meaningful comparison of hardware life.

Size doesn't matter .Its only for consumer comfort size is made universal not more than 120 mm.

If each and every company starts making discs of different sizes there we may face a situation of buying disc player ffor different sizes of discs

it can be less than 120mm but not more than that.

Conventional Blu-ray Discs contain 25 GB per layer, with dual layer discs (50 GB) being the industry standard for feature-length video discs. Triple layer discs (100 GB) and quadruple layers (128 GB) are available for BD-XL.The name Blu-ray Disc refers to the blue laser used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used forDVDs